1. Field of the Invention
The present invention relates to a magnetic transfer master carrier for magnetically transferring information to a magnetic recording medium, a magnetic transfer method using the magnetic transfer master carrier, and a magnetic recording medium in which information is magnetically transferred using the magnetic transfer master carrier.
2. Description of the Related Art
As magnetic recording media capable of recording information in a highly dense manner, magnetic recording media for perpendicular magnetic recording system (hereinafter referred to as perpendicular magnetic recording medium) are known. An information recording area of a perpendicular magnetic recording medium is composed of narrow tracks. Thus, a tracking servo technology for accurate scanning with a magnetic head within a narrow track width and for recording and reproducing a signal at a high S/N ratio is important to the perpendicular magnetic recording medium. To perform this tracking servo, it is necessary to record servo information, for example a servo signal for tracking, an address information signal, a reproduction clock signal, etc. as a so-called preformat, at predetermined intervals on the perpendicular magnetic recording medium.
As a method for preformatting servo information on a perpendicular magnetic recording medium, there is, for example, a method wherein a master carrier having a pattern consisting of a plurality of convex portions having a magnetic layer on their surfaces, which corresponds to servo information, is closely attached to the perpendicular magnetic recording medium, and then a recording magnetic field is applied thereto so as to magnetically transfer the servo information corresponding to the pattern of the master carrier to the perpendicular magnetic recording medium (see, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-203325 and 2000-195048 and U.S. Pat. No. 7,218,465, for example).
In this method, when the recording magnetic field has been applied to the master carrier closely attached to the perpendicular magnetic recording medium, a magnetic flux is absorbed into the patterned magnetic layer based upon the magnetized state of the master carrier. As a result, the recording magnetic field is increased in strength according to the pattern of the master carrier. The magnetic field increased in strength in the form of the pattern enables to magnetize only predetermined regions of the perpendicular magnetic recording medium. Thus, the servo information corresponding to the pattern of the master carrier is magnetically transferred to the perpendicular magnetic recording medium.
After the magnetic transfer, the recording magnetic field is cancelled, and the master carrier which has been closely attached to the perpendicular magnetic recording medium is separated therefrom.
Conventionally, magnetic materials with high saturation magnetization have been used as materials for magnetic layers of master carriers of this type. This is because when a recording magnetic field is applied, the magnetization of the magnetic layer of the master carrier is increased so as to easily absorb a magnetic flux in the magnetic layer.
However, the magnetic layer of the master carrier is very thin, specifically, roughly several tens of nanometers in thickness, thereby strongly influenced by a demagnetizing field. Therefore, even if a magnetic material with high saturation magnetization is used as the material of the magnetic layer, the strength of an effective magnetic field (recording magnetic field) applied to the magnetic layer decreases due to the demagnetizing field, and the magnetization of the magnetic layer becomes unsaturated. As a result, the magnetization of the magnetic layer cannot be increased as much as desired, causing a problem.
Moreover, it may be considered that the magnetic field to be applied is increased so as to achieve a suitable magnetization of the magnetic layer. However, when the magnetic filed to be applied is increased, a magnetic field present in regions other than the patterned magnetic layer of the master carrier may magnetize the perpendicular magnetic recording medium. This is also problematic.
Moreover, conventionally, a magnetic layer of such master carrier has high residual magnetization, and caused problems. As described above, even though the recording magnetic field is cancelled after the magnetic transfer, the residual magnetization is still present in the magnetic layer of the master carrier. In the case where the residual magnetization remains high, when the master carrier is separated from the perpendicular magnetic recording medium as described above, the position of the master carrier may be slightly shifted in a direction along the surface of the perpendicular magnetic recording medium. As a result, the residual magnetization in the magnetic layer of the master carrier may unnecessarily magnetize the perpendicular magnetic recording medium.